The use of ball grid arrays (BGAs) to package electronic circuits and devices such as semiconductor dies or integrated circuit chips is becoming more prevalent. BGA packaging has proven to provide substantial advantages over other packaging techniques such as, for example, dual in-line packages (DIPs), pin grid array (PGA) packages, tape carrier packages (TCPs), and quad flat packs (QFPs). The advantages of BGA packaging become especially significant when used to package dies or chips having a high pin count and when used to package devices employing high frequency signals. BGA packaging provides the additional advantage of being able to use conventional surface mount technologies (SMTs) and assembly techniques when mounting BGA packages to a printed circuit board (PCB).
A BGA package generally includes a die or chip, multiple substrate layers, and a heat spreader. The die is generally mounted on the heat spreader/stiffener using a thermally conductive adhesive or glue such as an epoxy. The heat spreader/stiffener provides thermal protection by increasing the conduction of heat away from the die to improve thermal performance. For example, absent appropriate heat removal mechanisms, the temperature within and around a typical die may increase to an undesirable level during normal operation, leading to reduced electrical performance or even failure of the device. The heat spreader/stiffener also provides structural and mechanical support by acting as a stiffener to provide added rigidity to the BGA package and thus may be referred to as a heat spreader/stiffener.
One of the substrate layers includes a signal plane that provides various signal lines or traces that can be coupled to a corresponding die bond pad using a wire bond. The signal traces are then coupled with a solder ball at the other end. As a result, an array of solder balls is formed so that the BGA package may be electrically and mechanically coupled to other circuitry, generally through a PCB, using the array of solder balls that is referred to as a ball grid array. Additionally, a ground plane will generally be included on one of the substrate layers to serve as an active ground plane to improve overall device performance by lowering the inductance, providing controlled impedance, and reducing cross talk. This is especially important when using BGA packaging with a die or chip having a high pin count or I/O count.
The fabrication of the ground plane is both expensive and time consuming but necessary to the overall performance of the device. The ground plane is generally constructed using a conductive metal such as copper and may include gold plating. These metals further add to the expense of fabricating the ground plane in a BGA package.
The complexity and cost of BGA packages are also influenced by the number of interconnections or vias that must be provided in the substrate layers to provide a path to connect each of the solder balls to either the ground plane, the power plane, or desired signal leads of the signal plane. In general, multiple substrate layers and multiple vias result in lower BGA package fabrication yields and higher costs. The formation of the vias create additional complexity and cost because each of the vias generally require the formation of a conductive layer, such as a metal layer on the internal walls of the via, to ensure a complete electrical path. This may be referred to as metallization. The metallization of the internal walls of each via increases the overall complexity and cost of manufacturing BGA packages.
Prior attempts at solving some of the problems mentioned above fail to satisfactorily solve the problem and suffer other disadvantages. For example, in U.S. Pat. Nos. 5,397,921 and 5,409,865 to Karnezos, a tape automated bonding grid array package is shown that uses a metallic heat spreader that also serves as a ground connection. The die of such a package must be mounted in a cavity of the heat spreader. The cavity is generally expensive to form or etch to a desired depth and contributes to the overall cost of the package. Also, the packaging configuration prevents the use of the heat spreader as a power plane or as a power connection point due to the dangers of an external short of the heat spreader.